The impact of riparian vegetation on bank erosion during extreme weather events

Riparian vegetation is a critical component of river ecosystems and plays an important role in sediment transport processes during floods. Riparian vegetation is commonly reported to stabilise riverbanks and reduce bank erosion; however, there is a lack of published data on bank erosion and riparian vegetation during extreme floods with significant out of channel flow. To reduce this knowledge gap, this study uses lidar and aerial imagery to investigate relationships between riverbank geomorphic change and riparian vegetation cover (i.e., trees and bushes) in the Uawa and Wairoa Rivers in New Zealand following extreme flooding during Cyclone Gabrielle in 2023. There was severe bank erosion in both rivers, with average bank material losses of −24.5 ± 5.9 m³/m for the Uawa River and −17.2 ± 5.5 m³/m for the Wairoa River, with average bank retreat of 3.99 m and 2.14 m, respectively. Geomorphic change was related to final vegetation cover and removal of riparian vegetation, with increased bank erosion where riparian vegetation was removed, compared to where it was retained. There were no clear links between bank erosion and river geomorphic units (i.e., inner, outer and straight banks), with more erosion of inner banks in the Uawa River, and more erosion of outer banks in the Wairoa River. Some of this variability was explained by differences in vegetation removal and retention; however, other factors, such as out of channel flow across floodplains (notably for inner bends) and woody debris, likely contributed to the variability of results, compared to meandering rivers where floods are confined within the channel banks.